1. Technical Field
This invention applies to rotor blades in general, and to apparatus for damping vibration within and cooling of a rotor blade in particular.
2. Background Information
Turbine and compressor sections within an axial flow turbine engine generally include a rotor assembly comprising a rotating disc and a plurality of rotor blades circumferentially disposed around the disk. Each rotor blade includes a root, an airfoil, and a platform positioned in the transition area between the root and the airfoil. The roots of the blades are received in complementary shaped recesses within the disk. The platforms of the blades extend laterally outward and collectively form a flow path for fluid passing through the rotor stage. The forward edge of each blade is generally referred to as the leading edge and the aft edge as the trailing edge. Forward is defined as being upstream of aft in the gas flow through the engine.
During operation, blades may be excited into vibration by a number of different forcing functions. Variations in gas temperature, pressure, and/or density, for example, can excite vibrations throughout the rotor assembly, especially within the blade airfoils. Gas exiting upstream turbine and/or compressor sections in a periodic, or “pulsating”, manner can also excite undesirable vibrations. Left unchecked, vibration can cause blades to fatigue prematurely and consequently decrease the life cycle of the blades.
It is known that friction between a damper and a blade may be used as a means to damp vibrational motion of a blade.
One known method for producing the aforesaid desired frictional damping is to insert a long narrow damper (sometimes referred to as a “stick” damper) within a turbine blade. During operation, the damper is loaded against an internal contact surface within the turbine blade to dissipate vibrational energy. One of the problems with stick dampers is that they create a cooling airflow impediment within the turbine blade. A person of skill in the art will recognize the importance of proper cooling air distribution within a turbine blade. To mitigate the blockage caused by the stick damper, some stick dampers include widthwise (i.e., substantially axially) extending passages disposed within their contact surfaces to permit the passage of cooling air between the damper and the contact surface of the blade. Although these passages do mitigate the blockage caused by the damper, they only permit localized cooling at discrete positions. The contact areas between the passages remain uncooled, and therefore have a decreased capacity to withstand thermal degradation. Another problem with machining or otherwise creating passages within a stick damper is that the passages create undesirable stress concentrations that decrease the stick damper's low cycle fatigue capability.
In short, what is needed is a rotor blade having a vibration damping device that is effective in damping vibrations within the blade and that enables effective cooling of itself and the surrounding area within the blade.